In the refining and petrochemical industry, various applications require the usage of high temperatures and pressures inside a reactor for either removing the sulfur or achieving other desired chemical reactions. Thus, the walls of the reactor have to withstand not only to the damaging characteristics of the chemicals inside the reactor but also to the changing conditions, i.e., temperature, pressure, etc. Special materials are used to build the reactors' walls, like stainless steel that includes various alloys.
For the past years, conventional low-alloy chromium-molybdenum 2.25 Cr-1 Mo steel has been extensively used for the reactor vessels. The reactors generally have been operated at temperature lower than 450° C. and with hydrogen partial pressure above 10 MPa. Growing demands for higher service temperatures/pressures imposed an increase of the reactor size, generating problems related to the construction, transportation and high temperature hydrogen attack during the reactor's service.
In order to solve at least this last problem, new generation of Vanadium modified Cr—Mo steel was developed and Nuovo Pignone (a business unit of General Electric located in Florence, Italy) fabricated the first 2.25 Cr 1 Mo 0.25V reactor for the petrochemical industry that withstands high temperatures and hydrogen pressures. This reactor has a wall thickness in excess of 250 mm, a diameter of up to 6 m, lengths up to 60 m, and a weight up to 2000 tons. FIG. 1 shows such a reactor 10. The reactor may be operated in high-temperature high-pressure hydrogen atmosphere. For efficiently carrying out, for example, the desulfurization reaction, the service temperature and pressure are increased, causing an increase in thickness and an overall scale up of the reactor dimensions. Thus, large parts 12 and 14 of the reactor 10 have to be welded together at joint regions 20.
The material (2.25 Cr 1 Mo 0.25V) of the walls of the reactor is used because it exhibits appropriate properties for hydrogen embrittlement, high temperature hydrogen attack and overlay disbonding, good toughness at low temperatures and improved resistance to temper embrittlement.
Because of the large size of the reactor, many parts making up the walls of the reactor have to be welded together as shown in FIG. 1. The welding process induces residual stresses in the joint areas due to the heat produced during this type of process, and the stresses are increased by high wall thickness.
FIG. 2 shows a closer view of a welded region 30, which includes parts 12 and 14 jointed together at the welding region 20. Those regions of the parts 12 and 14, whose properties are affected by the heat generated during the welding process are called heat affected zones (HAZ) and are indicated as regions 22 and 24. Thus, the heat-affected zone is the areas 22 and 24 of the base material that had its microstructure and properties altered by welding. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding the weld. The extent and magnitude of the change in properties depend primarily on the base material, the weld filler metal, and the amount and concentration of heat input during the welding process. To mitigate the stress induced during the welding process stress relieving heat treatments may be applied as indicated by American Petroleum Institute (API), American Society for Testing and Materials (ASTM), and American Society of Mechanical Engineers (ASME).
Combining (i) the residual stress formed during welding inside the walls of the reactor with (ii) the stress relieving heat treatment, results in the appearance of reheat cracking phenomena. Reheat cracking phenomena occurs primarily during the application of the heat treatment noted above, for example, in region 20 of FIG. 2. The reheat cracking occurs when grains at the boundary regions, during elevated temperatures, exhibit less or slightly weaker ductile properties than the grain located away from the boundary regions (creep failure damage mechanism).
Various tests exist for measuring the severity of the reheat cracking. One such test is the Geeble test, which provides qualitative indications about the ductility of a given structure. This test is based on the idea that the region most susceptible to hot cracking is the HAZ zone of the parent metal, in which contaminants entrapped at grain boundaries form liquid or low strength solid films while the grains become stiff and strong. It was also found that if such weak films exist over a large temperature range after solidification, the welded materials show hot cracks in the HAZ zone. To determine the range at which the welded HAZ zone is prone to hot cracking, a concept of nil strength temperature was introduced as the higher temperature of the brittle range, and appropriate attachments were designed to measure it. The lower temperature of the brittle range, so-called nil ductility temperature, was then taken as that at which 5% reduction in area on hot tensile samples appeared.
The Gleeble testing procedure requires a large number of samples to be hot tensile tested with strain rates representative of various welding methods (heat inputs). Thus, a simpler test, the Varestraint test was proposed and applied to study the hot cracking susceptibility of welded alloys. The Varestraint test includes bending a test plate while the weld bead is being made on the long axis of the plate. The original Varestraint test had some limitations, e.g., difficulty in controlling the real amount of strain at the outer bent surface due to the position of a neutral bending axis, which varied depending on the strength and strain partitioning between the hot and cold parts of the sample during bending.
However, the above discussed tests and others suffer from the fact that they provide only qualitative results and not selective responses of the damage causes, i.e., these tests are not able to reproduce the real heat treatments (in terms of time, temperatures and stress) that are used during fabrication.
Although these qualitative tests have been able in the past to ensure the quality of the fabrication processes, recent developments in Europe indicate that reheat cracking problems are surfacing for the 2.25 Cr 1 Mo 0.25V reactors and the existing tests are not enough anymore. Thus, it is desirable to support the manufacturing process of real components with a test method able to determine quantitatively, and not only qualitatively, the extension of the reheat cracking.
Accordingly, it would be desirable to provide systems and methods that are able to overcome the above noted limitations and provide tests for determining the susceptibility of a material to exhibit reheat cracks.